The Institute of Electrical and Electronics Engineers (IEEE) has defined the 802.3ah Ethernet in the First Mile (EFM) Point-to-Multipoint standard for Ethernet-based Passive Optical Networks (EPONs). These networks can act as optical access networks for residential and business subscribers, providing a full range of communications services to those users. Consistent with such deployments, the IEEE 802.3ah standard specifies optical wavelengths that leave room or capacity for communication services other than Ethernet, particularly broadcast video.
Unfortunately, key characteristics of Ethernet data transmission can cause significant optical interference to video signals through a phenomenon known as Stimulated Raman Scattering (SRS). The IEEE 802.3ah standard specifies that Ethernet data is transmitted to the subscriber using the 1490 mn optical wavelength. Wavelength division multiplexing (WDM) permits an optical network using the 1490 nm optical wavelength to propagate data to also deliver broadcast video on the same optical fiber using the 1550 nm optical wavelength. When the network transmits on two optical wavelengths simultaneously, such as the 1490 nm and 1550 nm wavelengths, it can be vulnerable to SRS. In IEEE 802.3ah EPON networks, the Ethernet signal transmitted at 1490 nm amplifies any video signal transmitted at 1550 nm, and therefore interference can result in a noticeable degradation of video quality. A particularly egregious case occurs when an Ethernet idle pattern is transmitted because no data is available to transmit. This causes extreme interference with broadcast video on certain channels.
FIG. 1 illustrates a conventional PON network 100 that is subject to SRS. Signals originate at a data service hub 110 and are transported on a passive optical network (PON) 120. The PON 120 comprises optical fibers 160 and 150, and an optical tap, or splitter 130, which divides the signals between a plurality of Subscriber Optical Interfaces 140. The Subscriber Optical Interfaces 140 are placed on the premises of each subscriber where they convert optical signals into the electrical domain in order to deliver video, voice, and data services to that subscriber. The PON 120 comprises a trunk fiber 160, which carries optical signals to a plurality of subscribers, and a drop fiber 150, which carries optical signals to a single subscriber. In some instances there can be an intermediate fiber which follows a portion of the optical splitting.
SRS between optical signals can develop in the trunk fiber 160, and is a function of the signal levels and optical wavelengths used. The amount of SRS optical interference introduced is a complex function of the distance 170 to the split. Very short lengths of optical fiber are not susceptible to SRS, but trunk fibers 160 of practical length tend to be quite susceptible to SRS. The IEEE 802.3ah EFM standard specifies distances of 10 and 20 km, which can be all in the Trunk Fiber 160, or some portion can be after the split, in the drop fiber 150. The worst case situation is where all the fiber is in the trunk portion 160. The IEEE 802.3ah standard also specifies the signal levels to be used.
FIG. 2 illustrates the related phenomenon of noise when a random signal is optically transmitted over a fiber, as it would manifest itself in a worst-case IEEE 802.3ah system. FIG. 2 illustrates the frequency spectrum of optical video signals. When an Ethernet idle pattern is transmitted, the signal power becomes concentrated at a few frequencies rather than being spread out evenly across the entire bandwidth as is the case illustrated in FIG. 2. This means that the idle pattern will affect fewer channels, but the effect will be much greater on those channels.
The curve plots the effect of SRS on an optical system carrying random data and also analog video. Better performance is reflected at higher points on the graph. The effect of random data is to worsen the carrier-to-noise ratio (C/N) of the received optical signal, to well below acceptable levels. The curve plots the C/N on each lower-frequency channel where the problem is the worst for a family of PONs of different practical lengths.
The figure also illustrates C/N limits for cable TV good engineering practice 220 and typical Fiber-to-the-Home (FTTH) typical performance absent SRS 210. If SRS causes the C/N to get significantly worse than the C/N without it 210, then the performance of the optical system will be degraded and users will not perceive the benefits that FTTH is supposed to offer. PONs with distances to the split 170 of 2 km 230, 5 km 240, 10 km 250 and 20 km 260 are shown. From this curve, one can see that a 2 km distance will not drop the C/N below cable TV good engineering practices 220, but it will be close at the lowest channel, and the performance will be worse than what a FTTH system should deliver. Longer PONs will cause unacceptable C/N performance on several channels.
One of ordinary skill in the art knows that a different selection of optical wavelengths could reduce or effectively eliminate the problem introduced by SRS. For example, other FTTH systems are known which use 1310 nm for bidirectional transmission of data. These systems are usually not troubled by SRS. However, the IEEE 802.3ah standard requires that downstream data be transmitted at 1490 nm, where the problem exists. It is possible to move the wavelength of the video transmission as high as possible in the 1550 nm window, but this will only result in slight improvement.
One of ordinary skill in the art is familiar with the specification for Gigabit Ethernet, which requires a prescribed bit pattern to be transmitted as an idle pattern when there is no data available to be transmitted. This method used in gigabit Ethernet and in certain other applications, is called 8B/10B encoding. The purpose of the 8B/10B encoding is to remove the low frequency dc component that digital optical systems are not able to transmit and to ensure clock synchronization to prevent the clock from wandering out of phase, which can damage data recovery. In 8B/10B encoding, for every 8 bits (one byte), a 10 bit code is substituted. The substituted 10 bit code is chosen to have very close to an equal number of 1s and 0s and three to eight transitions per symbol. The codes satisfy the requirement of no dc component in the signal, and the large number of transitions ensure clock synchronization. Furthermore, since a limited number of the available codes are used, the encoding provides another way to detect transmission errors.
The downside of 8B/10B encoding is that because 10 bits must be transmitted to represent 8 bits, the bandwidth required is increased by 25%. For instance, in a gigabit Ethernet system, the desired data is transmitted at 1 Gb/s, but because of 8B/10B encoding, the data rate on the fiber (the so-called wire rate) is 1.25 Gb/s. Furthermore, it has been found that when the idle pattern is transmitted and encoded with 8B/10B encoding, the resulting signal has strong power concentration at certain frequencies. These frequencies for Gigabit Ethernet happen to be at 62.5 MHz and all harmonics thereof, with the odd harmonics having virtually all of the power.
The IEEE 802.3ah standard defines two different idle codes. The first idle code, referred to as /I1/, has two versions. One version changes the running disparity on the link from positive (a preponderance of 1s—designated as /I1+/) to negative (a preponderance of 0s—designated as /I1−/), while the second version changes the running disparity from negative to positive. As known to one or ordinary skill in the art, the running disparity rules change the transmitted value from one column to the other based on certain rules related to the number of 1s or 0s that have been transmitted in the previous code group. These rules ensure that there is no dc content in the optical signal and that there is not a long string of like binary digits, thus ensuring reliable clock recovery.
The second idle code, referred to as /I2/, maintains the existing running disparity on the link. In a normal procedure for using these two idle codes the systems performs one of the following: (a) If, after the last transmitted frame, the link has a positive running disparity, the system transmits one /I1+/ to reverse the running disparity, and then transmits /I2/ continuously or (b) if, after the last transmitted frame, the link has a negative running disparity, the system transmits /I2/ continuously.
FIG. 3 is illustrates a measured spectrum 300 of a Gigabit Ethernet signal when it is carrying an idle pattern in the conventional art. On the spectrum 300, frequency in MHz is plotted along the x-axis, and relative amplitude in decibels (dB) along the y-axis. The amplitude scale shows increments of 10 dB. Note the strong presence of odd harmonics of 62.5 MHz. When this pattern is optically transmitted downstream in a network over an optical waveguide such as the one shown in FIG. 1, the SRS optical interference will cause very severe crosstalk in the video channels at these frequencies. The lowest frequency, 62.5 MHz, is of particular concern, as the worst SRS crosstalk usually occurs at lower frequencies such as this. Television channel 3 occupies this spectrum on the video layer. Its picture carrier is at 61.25 MHz (as prescribed by FCC frequency allocations), so the interference caused by the idle pattern appears 1.25 MHz above the picture carrier. One or ordinary skill in the art knows that this is a frequency at which the signal is particularly sensitive to interference. The interference will show up as a “beat,” or moving (usually) diagonal stripes in the picture.
In view of the foregoing, there is a need in the art to mitigate the effects of SRS optical interference on video transmissions in optical networks that use the IEEE 802.3ah data standard. Particularly, a need exists in the art for reducing or substantially eliminating the optical interference between data transmitted on a first optical wavelength and video information transmitted on a second optical wavelength when the data and video information are propagated along the same optical waveguide.